The invention is particularly applicable for inductively heating the outer teeth of a helical crankshaft gear of the type used on a motor vehicle and made from 4140 and 4150 steels to a hardened depth of about 0.05 inches uniformly distributed over the outwardly facing surfaces of the gear to produce uniform hardness on the irregular surfaces created by the various outwardly extending teeth. The invention will be described with a particular reference to this application; however, it is appreciated that the invention has much broader applications and may be used for hardening the inner or outer convoluted surfaces of various types of workpieces where the area to be hardened compared to the mass adjacent thereto is substantially less than the area compared to the adjacent mass at the protruding convolution, i.e. generally external gear teeth.
To withstand the wear and contact forces exerted during operation of a high power transmitting gear train, such an internal combustion engine or transmission, it is necessary to provide a hardened outer surface for the various gears constituting the gear train. In accordance with standard technology, the surfaces are hardened while the inner portion or core of the workpiece remains generally soft to present strength and ductility. For many years the surface hardness of gears has been accomplished by a carburizing process wherein the gears are first machined, then immersed in a carburizing media for a substantial length of time to infuse carbon into the surface, and then heat treated so that the carburized outer surface will have a substantially greater hardness than the inner portion or core of the gear. This type of process is lengthy and tremendously expensive. The carburization process does, however, produce gears having an inner tough unhardened mass or core with outer case hardened surfaces for the various teeth extending circumferentially around the outer periphery of the gear. Such costly carburizing processes have motivated many companies to attempt a direct adaptation of relatively inexpensive, easily controlled induction heating technology to the hardening of the outer teeth on gears. Many patents relate to attempts to accomplish this feat. Generally speaking, the only arrangement that has been at all successful has been machines which inductively heat and then quench harden only a few teeth at one time while the rest of the teeth are cooled for the purposes of preventing draw-back of previously hardened teeth. By indexing the induction heating mechanism of these machines about the total circumference of the gear, all of the teeth are successively hardened. In this manner, induction hardening of the gear geeth can be accomplished; however, the inductors were extremely complex and expensive. Such induction heating processes have been unsuccessful for mass production since they require a number of heating operations for processing a single gear. Further, such processes involved relatively complex indexing mechanisms and complex induction heating coils or inductors. Pfaffman U.S. Pat. No. 3,446,495 and Masie U.S. Pat. No. 4,251,704 illustrate the type of equipment wherein induction heating has been employed for the purpose of hardening the gear teeth on the circular periphery of a gear. These apparatus do function; however, they have the disadvantages previously described. Assignee of these two patents and other leading manufacturers of induction heating equipment have been seeking for many years an approach that can be used for inductively heating the outer peripheral surfaces of gears by using an encircling inductor so that the gears can be heated by the inductor and then quench hardened immediately thereafter to create case hardening on the outer surfaces of the gear without requiring any modification other than a certain amount of carbon in the steel itself to facilitate hardening of the outer surfaces. By developing such an induction heating concept, the time consuming, expensive carburizing process could be replaced by an apparatus for first inductively heating and then quench hardening the outer surface of the gears. A prior attempt to accomplish this goal is illustrated in Denneen U.S. Pat. No. 2,167,798 wherein a complex apparatus is provided for driving the current created by the inductor into the areas between adjacent teeth for the purposes of inductively heating and then immediately quench hardening the various gears at the same time. This process was not widely adopted and did not replace the carburizing process of gear teeth as previously described.
Immediately after the second World War, it was suggested that induction heating of the outer gear teeth could be accomplished by a dual frequency arrangement wherein a low frequency current would be used for preheating the gear teeth and then a high frequency current could be used for final heating preparatory to quench hardening. Two arrangements for applying this induction heating concept are illustrated in Jordan U.S. Pat. No. 2,444,259 and Redmond U.S. Pat. No. 2,689,900 wherein a single induction heating coil is provided with two frequencies for the purposes of accomplishing deep heating and then surface heating preparatory to quench hardening the teeth of a gear. This process was not successful. Another arrangement was suggested in Kincaid U.S. Pat. No. 2,590,546 wherein the gear is first placed in one induction heating coil driven by a relatively low audio frequency of less than about 15 KHz. Thereafter, the workpiece is shifted into another induction heating coil for heating by radio frequency. After radio frequency heating, the workpiece is shifted into a quenching ring for the purposes of quench hardening the outer heated teeth. This process has substantial merit in that relatively simple induction heating coils and quenching units can be employed for induction heating of the outer surfaces of the gear first by low frequency preheat and then by high frequency final heat to produce a skin effect for creating the hardness pattern around the gear teeth, as illustrated in FIG. 1 of Kincaid U.S. Pat. No. 2,590,546. Even though this process involves simple equipment and known technology, it has not been successfully employed for the purposes of mass producing hardened gears to absorb the stresses and forces created in high power gear trains, such as found in many heavily loaded gear drive trains such as transmissions. Even with these several suggestions on how induction heating can be employed for hardening the teeth of a gear, carburizing is still the basic and common way of accomplishing this hardening process.
Within the last few years, in view of the high price of gas, foreign competition requiring cost reduction and other market conditions, there is now a substantial, tremendous and immediate need for a successful process whereby induction heating of gear teeth can be used for the purpose of providing the gear teeth with hard tough high compression surfaces without causing brittle teeth or various under hardened teeth or over hardened areas between the teeth. To accomplish this objective, it is necessary and critical to produce an induction heating process wherein just before quench hardening the outer surfaces have a preselected temperature to a controlled depth whereas the material immediately behind or below the depth has a substantially lower temperature. Consequently, the quench hardening by liquid will quench harden only the outer surface to the controlled depth and not through harden the teeth. Induction heating of the gear teeth preparatory to quench hardening in the past has resulted in uneven heating and thus uneven hardness depth or pattern. Some of the surfaces have not been hardened at all, others have been hardened through the teeth and some have produced too deep or too shallow hardness at the root between the adjacent teeth. All of these nonuniformities in the hardness pattern are caused by nonuniform distribution of temperature gradients immediately before the liquid quench hardening. The liquid quenching causes rapid cooling. If the temperature is above the transformation temperature, hardening occurs. If the temperature is below the transformation temperature no hardening or reduced hardening occurs. Further, slow cooling prevents proper hardening. At this time, there is a substantial need for an invention in the induction heating field which will create a heat distribution around the teeth of a gear immediately before liquid quench hardening which is uniform so that the resulting hardness pattern after quenching will be uniform. In addition, this induction heating process must be capable of performance at a high rate necessary to substantially reduce the cost required in hardening gear teeth over the cost involved in the processing and equipment now used for carburizing and must use easily controlled simplified inductors.